We are interested in studying how cells recognize and discriminate between self and nonself. This process is relevant to human health because failure to discriminate between self and nonself can lead to recurrent infections, cancer and autoimmune diseases. Moreover, the recognition of nonself can lead to graft rejection making it relevant for tissue transplant. The social amoeba Dictyostelium possess a self/nonself recognition mechanism based on cell adhesion and signaling that promotes the preferential aggregation and sporulation of genetically related individuals. Our long term objective is to study and identify the molecular mechanisms that mediate self/nonself recognition in Dictyostelium discoideum. We have evidence that the transmembrane proteins LagC and LagB are involved in self/nonself recognition and we hypothesize that they interact functionally to mediate allorecognition through allele-specific intercellular adhesion and signaling. We are proposing the following specific aims to test our hypothesis. First, we will determine whether LagB and LagC are binding partners by utilizing a gene-specific yeast-2-hybrid assay, immunofluorescence and co-immunoprecipitation techniques. Moreover, we will conduct epistatic analysis of lagC and lagB by characterizing the phenotypes of lagC lagB double null mutants. Second, we propose to investigate the role of LagB and LagC in self/nonself recognition by utilizing a gene replacement system that allows us to replace wild type sequences of lagC or lagB with polymorphic lagC and lagB sequences from D. discoideum isolates collected from the wild. We anticipate that by replacing the wild type gene with a polymorphic variant we will disturb the self/nonself recognition mechanism. We will test whether the expression of a polymorphic allele fails to complement the null allele in the wild type strain. We will also investigate whether replacing lagC and lagB with a matching pair from a single wild strain would complement the double null. Third, we will identify other members of the LagC/LagB signaling pathway by conducting a suppressor screen of lagB and lagC. We will also attempt to identify other potential proteins involved in the allorecognition process by following a candidate approach and surveying other proteins in the LagC multigene family. This investigation is relevant to public health because studying self/nonself recognition in a simple model organism will allow us to uncover the mechanisms underlying this complicated process. This will give us new tools in generating therapies for ailments, such as cancer and autoimmune diseases, in which the process of distinguishing between self and nonself is impared.